The present invention relates generally to the art of solid state relays for protecting electrical components such as electric motors. More particularly, the invention relates to a circuit and a method for automatically resetting a solid state relay a predetermined time after a trip to permit equipment coupled to the relay to resume operation.
In recent years a number of improved protection devices have become available for electrical machines such as single and polyphase motors. Such devices typically offer protection against electrical system malfunctions such as overload, short circuit, phase loss and the like that would otherwise cause permanent damage to the machines. As advancements in the design and manufacture of solid state circuitry have appeared, conventional overload protection devices have increasingly been designed to incorporate such circuitry, thereby often reducing the unit cost of the devices while providing enhanced and more reliable performance.
Commercially available overload relays, for example, currently include extremely rapid solid state analog or digital circuitry capable of interrupting power to a protected device in the event of overload, phase loss, ground fault or other failure. Solid state overload relays of this type are commonly available in relatively compact, affordable packages that can be easily installed and serviced. In addition to circuitry for detecting fault conditions, such relays also commonly include power supply circuitry for storing energy from the load circuit being controlled. The power supply circuitry is coupled to an interrupter coil, permitting the device to energize the coil and interrupt power to the load without the necessity of a separate, independent power supply. Such packages typically also include mechanical or electro-mechanical hardware permitting an operator to manually reset the device or to verify its operability, such as by depressing a test button. An overload relay generally of this type is described in U.S. Pat. No. 5,332,986, issued to Christopher Wieloch on Jul. 26, 1994 and assigned to Allen-Bradley Company, Inc. of Milwaukee, Wis.
In addition to failure detection and power circuitry, it is often desirable in overload protection devices, such as overload relays, to provide circuitry for resetting the device to a conducting state once a predetermined time has passed after a trip. In earlier mechanical devices, a bimetallic switch or similar arrangement was used to mimic the cooling of the protected machinery, resetting the device automatically after the switch had sufficiently cooled to close contacts in the relay. More modem devices commonly employ digital technology, typically including a programmed microprocessor to model cooling of the machinery, automatically resetting the device based upon the temperature predicted by the model. However, for lower power or less critical applications, digital devices of this type are often prohibitably expensive.
There is a need, therefore, for an automatic reset circuit, particularly for solid state overload relays, comprised of analog circuit components that can be manufactured at substantially lower cost than programmable digital circuitry adaptable to carry out an automatic reset function. In particular, there is a need for an automatic reset circuit for such relays that stores energy for driving a reset coil from the same power circuit used to drive the relay trip coil. Moreover, such a reset circuit should advantageously include a minimum number of circuit components and be susceptible to mass production by known integrated circuit manufacturing techniques.